[GlobalMerge] Allow merging globals with explicit section markings.

At least on ELF, it's impossible to tell from the object file whether
two globals with the same section marking were merged: the merged global
uses "private" linkage to hide its symbol, and the aliases look like
regular symbols. I can't think of any other reason to disallow it.
(Of course, we can only merge globals in the same section.)

The weird alignment handling matches AsmPrinter; our alignment handling
for global variables should probably be refactored.

Differential Revision: https://reviews.llvm.org/D49822

llvm-svn: 338791